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A Molecular Approach for Dechiphering and Designing Cobalt Water Oxidation Catalysts

  • Author(s): Nguyen, Andy
  • Advisor(s): Tilley, T. Don
  • et al.

Chapter 1:

Efficient C-H Bond Activations via O2 Cleavage by a Dianionic Cobalt(II) Complex

A dianionic, square planar cobalt(II) complex reacts with O2 in the presence of acetonitrile to give a cyanomethylcobalt(III) complex formed by C–H bond cleavage. Interestingly, the group-transfer reagents PhIO, IO4-, and p-tolyl azide react similarly to give the same cyanomethylcobalt(III) complex. Competition studies with various hydrocarbon substrates indicate that the rate of C–H bond cleavage greatly depends on the pKa of the C–H bond, rather than on the C–H bond dissociation energy. Kinetic isotope experiments reveal a moderate KIE value of ca. 3.5 using either O2 or PhIO. The possible involvement of a Co(IV)-oxo species in this chemistry is discussed.

Chapter 2:

Mechanistic Investigations of Water Oxidation by a Cobalt Oxide Analogue: Evidence for a Highly Oxidized Intermediate and Exclusive Terminal Oxo Participation

Artificial photosynthesis (AP) promises to replace our dependence on fossil energy resources via conversion of sunlight into sustainable, carbon-neutral fuels. However, large-scale AP implementation remains impeded by a dearth of cheap, efficient catalysts for the oxygen evolution reaction (OER). Cobalt oxide materials can catalyze the OER and are potentially scalable due to the abundance of cobalt in the earth’s crust; but unfortunately, the activity of these materials is insufficient for practical AP implementation. Attempts to improve cobalt oxide’s activity have been stymied by limited mechanistic understanding that stems from the inherent difficulty of characterizing structure and reactivity at surfaces of heterogeneous materials. While previous studies on cobalt oxide revealed the intermediacy of the unusual Co(IV) oxidation state, much remains unknown, including whether bridging or terminal oxo ligands form O2 and what the relevant oxidation states are. We have addressed these issues by employing a homogenous model for cobalt oxide, the [CoIII4] cubane (Co4O4(OAc)4py4, py = pyridine, OAc = acetate), that can be oxidized to the [CoIVCoIII3] state, and upon addition of one equivalent of sodium hydroxide, the [Co(III)4] cubane is regenerated with stoichiometric formation of O2. Oxygen isotopic labeling experiments demonstrate that the cubane core remains intact during this stoichiometric oxygen evolution reaction, implying that terminal oxo ligands are responsible for forming O2. The OER is also examined with stopped-flow UV-visible spectroscopy, and its kinetic behavior is modeled, to surprisingly reveal that O2 formation requires disproportionation of the [CoIVCoIII3] state to generate an even higher oxidation state, formally [CoVCoIII3] or [CoIV2CoIII2]. The mechanistic understanding provided by these results should accelerate the development of OER catalysts leading to increasingly efficient AP systems.

Chapter 3:

Prediction of redox potentials for Co4O4 cubanes over a 1400 mV range: Implications for the feasibility of formal Co(V)¬–Oxo species

Synthetic methods were developed for generating new Co4O4 cubanes comprising highly electron-deficient ligands, electron-rich ligands, and ligands with secondary hydrogen-bond donors. The synthetic methods also allowed for cubanes with mixed carboxylates. A two-dimensional linear free energy relationship (LFER) was found to accurately predict the [CoIII4O4]4+/[CoIVCoIII3O4]5+ and [CoIVCoIII3O4]5+/[CoIV2CoIII2O4]6+ redox potentials as a function of the pKa of the ligands bound to the cubane. These redox potentials span a maximum of 1420 mV. Comparing cubanes containing intramolecular hydrogen-bonds with those lacking hydrogen-bonds allowed for the rationalization of the different redox potentials previously seen for the same cubane in water versus polar aprotic solvents. Finally, application of the LFER to the proposed, non-isolable intermediates in cubane-mediated water oxidation allowed estimation of their redox potentials and support for their plausibility.

Chapter 4:

Tunable, site-isolated Co4O4 oxygen-evolution catalysts uniformly dispersed within porous frameworks

Three-dimensional porous coordination-polymers composed of Co4O4 cubanes and several organic linkers were synthesized as new, tunable heterogeneous oxygen evolution reaction (OER) catalysts. Frameworks were synthesized either by substituting the acetate ligands of the cubane, Co4O4(OAc)4py4 (OAc = acetate, py = pyridine), with tricarboxylic acids, or by substituting the pyridine linkers with tripyridyl ligands. Although these solids lack long-range periodicity, their structures were well characterized by a suite of techniques, including X-ray absorption spectroscopy (XAS), extended x-ray absorption fine structure (EXAFS), and pair-distribution function analysis (PDF). Evidence for their high surface areas come from thermogravimetric analysis and gas sorption data. The tripyridyl-linked frameworks were significantly more stable than the tricarboxylate-linked frameworks towards the high pH conditions used in the OER. Stoichiometric O2 evolution occurs by addition of the oxidized coordination-polymers into pH 14 solutions, demonstrating the retention of the molecular cubane precursor’s OER reactivity. Finally, thin films of these polymers on glassy carbon perform as OER electrocatalysts. Their overpotential is tuned by the basicity of the linker ligands, demonstrating these as tuneable heterogeneous OER electrocatalysts.

Chapter 5:

Carboxylate-Supported Mixed Manganese-Cobalt Oxido Cubane and Dangler Clusters

In a single step, manganese(IV) is readily incorporated into an oxido-cobalt(III) scaffold by rational, self-assembly of permanganate, cobalt(II) acetate and pyridine to form the cubane oxo cluster MnCo3O4(OAc)5py3 (OAc = acetate, py = pyridine), 1-OAc. The electronic properties of the cubane are tuned by exchange of the μ1-acetate ligand for Cl– (1-Cl), NO3– (1-NO3), and pyridine ([1-py]+). Addition of a fifth Co(II) to 1-OAc forms MnCo4O4(OAc)6(NO3)py3 (2), reminiscent of the “dangler” motif of photosystem II. The electronic states of these clusters were examined by electron paramagnetic spectroscopy (EPR) and SQUID magnetometry. Cyclic voltammetry of the clusters in organic solvents demonstrates an accessible [MnCo3O4]5+/[MnCo3O4]6+ redox couple that is modulated by the nature of the ligand on Mn. Yet, DFT showed redox process occur at a Co-ion rather than the Mn-ion. Interestingly, the addition of the dangler ion gave four accessible redox states, perhaps suggesting a role for the dangler ion found in photosystem II.

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